Nuclei of the first kind usually have spin quantum number equal to one half, and in particular include the nuclei .sup.1 H, .sup.19 F and .sup.31 p which can give readily detectable NMR effects in appropriate conditions. In a magnetic field such nuclei may precess around the direction of the field with a frequency given by EQU .omega.=.gamma.B where .omega.=2.pi.f,
f is the resonant frequency, .gamma. is the gyromagnetic ratio of the nuclei and B is the strength of the applied magnetic field. If the nuclei are irradiated with electromagnetic waves at or very close to their resonant frequency f, they can exchange energy with the electromagnetic waves by changing the angle between the direction of their magnetic moment and the direction of the applied magnetic field. When such irradiation ceases the nuclei tend to return towards an alignment parallel to the applied magnetic field, emitting radiation of frequency f. This proceeds as a relaxation process with a time constant called the spin-lattice relaxation time T.sub.1 which depends on the processes available for the loss or conversion of their excess energy. The resonant frequency is proportional to the field strength, so it can be tuned to a specified frequency by suitably adjusting the applied magnetic field strength.
Nuclei of the second kind have a spin quantum number greater than one-half and may include for instance .sup.7 Li, .sup.9 Be, .sup.11 B, .sup.14 N, .sup.23 Na, .sup.27 Al, .sup.35 Cl, .sup.39 K, .sup.55 Mn, .sup.59 Co, .sup.75 As, .sup.79 Br, .sup.81 Br, .sup.127 I, .sup.197 Au, .sup.209 Bi. They can show detectable nuclear quadrupole resonance effects at resonance frequencies which are mainly determined by the nature of the nuclei and the sub-molecular environment which they are situated. In any compound, the nature and disposition of adjacent nuclei and electrons produce sub-molecular electric field gradients which interact with the nuclear electric quadrupole moments of the nuclei so as to determine one or more quadrupole resonance frequencies. These quadrupole resonance frequencies therefore depend on and indicate the chemical environment as well as the nuclear species involved. The quadrupole resonances are modified by temperature and pressure and may be shifted by an externally applied magnetic field to an extent dependent on the gyromagnetic ratio of the quadrupolar nuclei involved. For some nuclei (e.g. .sup.14 N) this gyromagnetic ratio is small and hence the chemical environment the main factor determining the quadrupolar resonances in relatively low applied magnetic fields.
Where nuclei of the first kind are also present, particularly if their gyromagnetic ratios are much larger than those of any quadrupolar nuclei present, the applied magnetic field strength can be readily adjusted to make the NMR frequency of the first nuclei equal to an NQR frequency of the second nuclei. This is called a level-crossing field strength; in such conditions energy can readily be exchanged between the first nuclei and the second nuclei, and this changes the span-lattice relaxation time constant T.sub.1 of the first nuclei significantly. In some cases this time constant may be reduced by several orders of magnitude by setting the applied magnetic field to a level-crossing field strength. In other cases the effect may be much less but still substantial enough to be used as the basis of methods for detecting the presence of specific substances.